The present invention relates to a developing apparatus for developing an electrostatic latent image formed on a photosensitive layer into a visible image and, more specifically, to a developing apparatus for developing an electrostatic latent image into visible image, without coming into contact with the photosensitive layer.
Many processes for developing electrostatic latent images are conventionally known, such as the dry development processes including the magnetic brush process, the cascade process, the fur brush process, etc.; the liquid development process using electrophoresis; and various modifications of these processes.
Recently, electrophotographic recording apparatuses of a light beam scanning type, called laser printers or liquid crystal printers, have been developed and are coming into wide use. Many of the electrostatic latent images used in these electrophotographic recording apparatuses are developed by the reversal development process.
The reversal development process is one in which a light beam is applied to the surface of a photosensitive layer, so that only those portions of the photosensitive layer at which electrostatic charges are erased are developed. According to the reversal development process, the area of the regions exposed to the light beam can be relatively small, so that the load of the light source is reduced, and the accuracy required for mechanical beam control lessened.
According to the reverse development process (hereinafter referred to as the non-contact development process), an electrode 3 having a thin layer of a so-called one component type developing agent 2 thereon is opposed to an electrostatic latent image forming surface 1, or a surface to be developed, in an non-contact relationship, as shown in FIG. 1A, and the developing agent 2 is electrostatically flown and attracted to an electrostatic latent image on the latent image forming surface 1, by the agency of an electrical field generated between the electrostatic latent image and the electrode 3, with a bias voltage being applied externally. At present the non-contact developing process not only in the reversal developing device but also in the ordinary developing device (e.g., a copying device) are on trail.
Under the circumstances, however, there is neither means for uniformly charging the developing agent 2 nor effective means for maintaining a predetermined charge amount to selectively fly the developing agent 2 in the non-contact development. Moreover, the electric field required for the flight of the developing agent 2 is too large. Therefore, the latent image requires a potential of about 1,000 V even through the gap D between the latent image forming surface 1 and the electrode 3 is narrowed to, for example, 150 microns. Photosensitive materials to resist the voltage of 1,000 V are limited in number. Even if an external bias voltage is applied in order to reduce the strength of the aforesaid potential, arc discharge will be caused between the electrode 3 and the latent image forming surface 1 to damage some parts of the apparatus, since the electrode 3 is in close vicinity to the latent image forming surface 1. Further, the developing agent 2 is charged unevenly, so that fog or streaks may be caused. These problems are left unsettled.
For keeping the charges on the developing agent 2, a toner may be separated from a two-component developing agent. According to this method, however, arc discharge is more liable to be caused if a carrier is mixed in the toner in the developing process, failing to be completely separated. Also, it is to be desired that the non-contact development should be effected by the use of a one-component developing agent containing no carrier.
The primary advantage of the non-contact development process lies in the fact that previously developed images will never be disturbed by superposed development in color electrophotography. For color developing agents to provide various colors, therefore, it is not advisable to use magnetic toners which contain black magnetic powder. Thus, the developing agent used in expected to be a nonmagnetic, one-component developing agent (hereinafter referred to simply as a toner).
In a currently prevalent method for uniform toner charging, the toner is rubbed against a developing roll by means of a rubber blade, or the like, to form a thin layer, when the toner is charged by friction with the developing roll or blade. The efficiency of contact between the toner and the roller or the blade is very low. It is therefore almost impossible to apply uniform charges to the toner forming the toner layer. This is a cause of the production of the defective images.
The theory of the non-contact development will be described in due order. It was revealed that the most dominant force in constraining the toner's flight is a reflected image force Fm produced between the toner and the developing electrode 3. This decision was made after the relationship between the magnitude of an electric field necessary for the toner's flight and the charge amount was examined, using toners of different charge amounts and providing a dielectric layer 3-1 on the developing electrode, as shown in FIG. 1C. If a toner particle with radius r has an electric charge q in the center, as shown in FIG. 1B, then the reflected image force Fm is given by ##EQU1## where .epsilon..sub.0 is the dielectric constant of a vacuum. As seen from this equation, the reflected image force Fm varies in inverse proportion to the square of the toner particle diameter or of the distance between the toner and the electrode 3. Hereupon, if the force to attract the toner to the electrostatic latent image or the flying force of the toner produced in the gap D of the developing region by an electric field E generated by the electrostatic latent image is F.sub.D, we have a relationship ##EQU2## This relationship is the flying condition of the toner. The flying force F.sub.D has a maximum to ensure the easiest flight when the toner particle diameter is as great as possible, and when the charge amount takes a value q=8.pi..epsilon..sub.0 r.sup.2 E based on a conditional expression, ##EQU3## which is obtained by differentiating both sides of the above equation. The charge amount of the toner may be adjusted in some measure by suitably selecting the toner material. As described before, however, it has conventionally been impossible to charge the individual toner particles uniformly.
In view of these circumstances, it may be seen that the developing sensitivity may be improved, or the toner's flight may be facilitated, by keeping the toner away from the electrode 3. As shown in FIG. 1C, for example, the dielectric layer 3-1 of polyester or epoxy resin with a thickness of 10 to 20 microns may be put on the electrode 3. The reflected image force Fm can be drastically reduced by the dielectric layer 3-1. Thus, the toner's flight is taken to be actually improved. The dielectric layer 3-1 cannot, however, avoid frictional charging between itself and the toner. As a result, the uniformity and stability of an image produced will be greatly damaged by the interference of an electrostatic force newly produced between the toner and the dielectric layer 3-1.